An example of virtual sound localization technology is a method of achieving localization of virtual sounds in front of and behind a listener using an HRTF. With this method, virtual sounds are generated as follows.
First, a measurement speaker is placed at a desired position at which a virtual sound (hereinafter, also referred to as a virtual sound source) is localized, and an HRTF is measured from this measurement speaker to the entrance of the external ear canal of the listener. This measured HRTF is used as a target characteristic.
Next, an HRTF from a reproduction speaker used for virtual sound localization to a listening position is measured by reproducing a reproduction sound source. The HRTF measured in this way is used as a reproduction characteristic.
Here, the measurement speaker placed at a position where a virtual sound is to be localized is used only for measuring a target characteristic. Therefore, the measurement speaker is not used afterward when the listener reproduces a reproduction sound source. Only the reproduction speaker is used for localization of a virtual sound source by reproducing a reproduction sound source.
Then, an HRTF for virtual sound localization is calculated using the target characteristic and the reproduction characteristic. The calculated HRTF is used as a filter characteristic. The filter characteristic is convoluted into the reproduction sound source, thereby generating a reproduction sound source which the listener hears as if a sound were output from a virtual speaker.
When a virtual sound is generated in the above manner, as reproduction speakers used to reproduce reproduction sound sources, (1) front speakers placed in front of the listener are used as typified by a front virtual surround system, (2) near-ear speakers placed near the ears of the listener are used as typified by a headphone virtual surround system, and (3) front speakers placed in front of the listener and near-ear speakers placed near the ears of the listener are combined, and both types of the speakers are used.
Patent Literatures (PTLs) 1 and 2, for instance, disclose a system in which both a front speaker and a near-ear speaker are used.
For example, PTL 1 discloses a game machine body which has an expanded capability terminal. This expansion terminal has a sound output function. Further, the game machine body is connected to a television receiver which includes a speaker. This game machine body causes sound to be output from the television receiver, and also sound to be output from headphones connected to the expanded capability terminal.
Further, this game machine body has a function of giving, to a headphone reproduction signal, a time delay from when a sound is reproduced by a speaker to when the sound wave thereof reaches a listener. Specifically, the game machine body makes adjustment so that the listener hears the sound from the speaker and headphone reproduction signals from the headphones, simultaneously.
The above configuration allows a user to simultaneously listen to sound from the television receiver and sound from the headphones, thus achieving reproduction and creation of more realistic sound than before. Furthermore, a time delay is given to a headphone reproduction signal, thereby bringing a sound localization position close to or away from the listener.
Further, PTL 2 discloses a technique of improving the localization precision of an audio channel localized at a rear position in particular, by using both types of speaker, namely, front speakers and near-ear speakers.